Introduced by M. E. Hines in the IEEE Transaction on Microwave Theory and Techniques, Vol. MTT-19, No. 5, May 1971, the edge-guided mode has been utilized in ferrimagnetic stripline circuits, in which wave energy is concentrated along one edge thereof. M. E. Hines made use of this phenomenon in realizing an isolator of wider band characteristics by installing a resistance element at one end of the line conductor. This isolator, however, is intricate in construction because of the resistance element used. Furthermore, in this type of isolator, the greater the isolation desired, the larger the required size of the resistance element, with the result that the overall size of the isolator becomes inevitably enlarged.
Another prior art isolator, proposed by K. Araki et al in The Institute of Electronics and Communication Engineers of Japan, Microwave Conference, Paper No. MW74-20, June 1974, has desirable characteristics, which have been achieved by grounding one end of the line conductor, instead of using Hines' resistance element. This isolator, however, must be fabricated three-dimensionally because the line conductor is of three-dimensional construction. Furthermore, the isolator can operate only within a comparatively narrow frequency band.